Steps to reproduce:
1. Download and open the attached project
2. Open the profiler
3. Build for Standalone
4. Launch the build in windowed mode with 768p resolution or higher.
5. Move mouse in and out of window constantly
6. Obseve profiler for EarlyUpdate.ProcessMouseInWindow

Expected result: should not take almost any time to check whether mouse is in window
Actual result: process sometimes spikes up to 3-4 ms

Reproduced on 5.6.2p4*, 2017.1.0f3, 2017.2.0b3
Not reproduced on 5.5.4f1
Regression introduced in 5.6

*5.6 mentions the time as Unallocated time spent, but most likely the same cause.

Verified FIXED 2017.3.0b3, 2018.1.0a1